In this type of exhaust purification apparatus, an additive injected into an exhaust gas of an internal combustion engine is supplied to a catalytic device, which then purifies the exhaust gas by using the additive. In order for the catalytic device to purify the exhaust gas with high efficiency, two requirements need to be fulfilled, one being that the additive should be satisfactorily mixed with the exhaust gas, and the other being that the exhaust gas mixed with the additive should be supplied as uniformly as possible to substantially the entire inlet area of the catalytic device. Various measures have hitherto been taken to meet the requirements, and one of the measures is described in, for example, Patent Document 1.
In the exhaust purification apparatus disclosed in Patent Document 1, a casing containing a DPF is arranged in an exhaust passage of an internal combustion engine. The casing has a large-diameter part located downstream of the DPF, and a vane plate for producing a swirl flow is arranged in the large-diameter part of the casing. The large-diameter part having the vane plate arranged therein is connected to a downstream-side small-diameter part through a flow contraction part with a gradually decreasing diameter, and a fuel nozzle serving as additive injection means is arranged in the small-diameter part. A NOx absorption catalytic converter as the catalytic device is arranged downstream of the small-diameter part.
In such exhaust purification apparatus, the exhaust gas from the engine flows through the DPF, the large-diameter part, the flow contraction part and the small-diameter part in the mentioned order. As the exhaust gas passes through the NOx catalytic converter, the NOx in the exhaust gas is absorbed in the NOx catalytic converter, and when the amount of the NOx absorbed reaches a predetermined value, NOx purge is executed to release the NOx from the NOx catalytic converter to be reduced. During the NOx purge, fuel injected from the fuel nozzle is supplied, together with the exhaust gas, to the NOx catalytic converter, and by using the fuel as a reducing agent, the NOx catalytic converter releases the NOx absorbed therein so that the NOx may be reduced. When the exhaust gas flows into the large-diameter part, a swirl flow of the exhaust gas is generated by the vane plate, thus furthering the mixing of the fuel with the exhaust gas.
The measure adopted in the exhaust purification apparatus of Patent Document 1 is focused on only one of the aforementioned two requirements (i.e., promotion of the mixing of fuel with the exhaust gas). Other exhaust purification apparatus have also been proposed in which measures are taken to meet both of the two requirements.
FIG. 4 illustrates the configuration of a related exhaust purification apparatus relating to published application USPGP 2009/0019842. As shown in FIG. 4, a selective reduction NOx catalytic converter 16 serving as a catalytic device is arranged downstream of a DPF 15. A venturi-shaped mixing chamber 13 is disposed between the DPF 15 and the NOx catalytic converter 16 and includes a diameter-reducing section 13a having a diameter decreasing toward the downstream side, a constricted section 13b continuously extending from the diameter-reducing section and having the smallest diameter, and a diameter-increasing section 13c continuously extending from the constricted section and having a diameter increasing toward the downstream side. A vane plate 18 for generating a swirl flow is arranged in an upstream portion of the diameter-reducing section 13a, and a urea water injection nozzle 19 serving as additive injection means is arranged immediately downstream of the vane plate 18.
The exhaust gas passed through the DPF 15 flows through the vane plate 18 in the diameter-reducing section 13a of the mixing chamber 13, whereby a swirl flow of the exhaust gas is produced. Also, urea water is injected from the injection nozzle 19 into the exhaust gas. As the exhaust gas flows through the diameter-reducing section 13a whose cross-sectional flow area gradually decreases, the flow of the exhaust gas narrows, so that the urea water is mixed satisfactorily with the exhaust gas. Subsequently, the exhaust gas flows through the constricted section 13b into the diameter-increasing section 13c. While the exhaust gas flows in this manner, the urea water in the exhaust gas is hydrolyzed due to heat of the exhaust gas, thus producing ammonia (NH3). When passing through the diameter-increasing section 13c, the exhaust gas diffuses, and the ammonia is dispersedly supplied to the inlet of the NOx catalytic converter 16 arranged downstream-side of the diameter-increasing section 13c. Using the ammonia thus supplied, the NOx catalytic converter 16 selectively reduces the NOx in the exhaust gas.    Patent Document 1: Unexamined Japanese Patent Publication No. 2006-183509